In manufacturing a semiconductor device or a flat panel display (FPD), there has been widely used a plasma processing apparatus for performing, e.g., an etching, a depositing, an oxidizing and a sputtering process by using a plasma. Generally, in the plasma processing apparatus, a radio frequency power supply unit applies a radio frequency power to a radio frequency electrode disposed inside or outside a processing vessel or chamber. The radio frequency power supply unit includes an oscillator or a radio frequency power source for outputting a radio frequency power and a matching unit for matching an impedance at the side of a load (electrode, plasma and chamber) with an impedance at the side of the radio frequency power source. Since the radio frequency power source is typically designed as a pure resistance output of 50Ω, the impedance of the matching unit is set or adjusted such that the impedance at the load side including the matching unit becomes 50Ω (see, e.g., Japanese Patent Laid-open publication No. 11-61456).
In general, such type matching unit includes one or more variable reactance elements such as a variable capacitor and a variable inductance coil, and serves as a variable matching unit capable of variably adjusting the impedance in the matching unit or the load impedance by selecting each step or position within a variable range with, e.g., a stepper motor. Further, during a plasma processing, if a plasma impedance is changed due to a pressure variation, the matching unit variably adjusts the impedance position of the variable reactance elements to make the load impedance automatically coincide with the matching point 50Ω. In order to carry out such auto-matching, there are used circuits for measuring the load impedance and a controller for variably controlling the impedance positions of the variable reactance elements by using the stepper motor such that the measured load impedance coincides with the matching point 50Ω.
In the conventional plasma processing apparatus, the impedance position of the matching unit before or upon a plasma ignition is set to coincide with a position corresponding to the matching point or a position close to that. However, when the gas pressure in the chamber is low, the plasma is difficult to get ignited. Such trend becomes particularly conspicuous in a parallel plate type plasma processing apparatus.
As an approach for dealing with the problem that the plasma is not to get ignited, there has been proposed in the Japanese Patent Laid-open Publication No. 11-61456 a method wherein the radio frequency power starts to be applied under a higher pressure than a level required for the plasma processing and then the pressure is lowered to the required level after the plasma ignition. However, such a method is cumbersome in that an optimal initial pressure for the plasma ignition should be set for every process, and further it is not assured that the plasma will be securely ignited under an initial pressure slightly higher than the required level. Meanwhile, if the initial pressure is much higher than the required level, the transition of the pressure after the plasma ignition may affect the process.